This invention relates generally to data communications and, more particularly, to a communication unit for exchanging serial data.
Known communication systems are extensively used with many types of electronic equipment. The communications systems are used to transmit serial data between two pieces of electronic equipment. For example, some known computers utilize communication systems to transmit serial data back and forth between two computers at high rates of speed. The serial data is typically transmitted between the two computers using either a synchronous or asynchronous communication system.
In synchronous communication systems, clock pulses and data are simultaneously transmitted from the first piece of equipment to the second piece of equipment over a cable. The clock pulses from the first piece of equipment establish the speed at which the data will be exchanged. The second piece of equipment that is responding to the first piece of equipment uses the edges of these clock pulses to recover the data sent from the first piece of equipment and also to time the return of data to be sent to the first piece of equipment. During transmission of the data from the first piece of equipment to the second piece of equipment, the clock pulses and data undergo equal delays due to length and loading of the cable. As a result, the phase relationship of the data and clock pulses sent by the first piece of equipment is preserved at the second piece of equipment. When the data link direction is reversed and data is sent from the second piece of equipment to the first piece of equipment, the cable delay not only delays the clock as it propagates from the first piece of equipment to the second piece of equipment, but also delays the data the same amount as it propagates from the second piece of equipment back to the first piece of equipment. The first piece of equipment, having only the original clock pulses with which to sample the returned data, is presented with data which is delayed an unknown amount of time, this time being a function of the cable loading and length. As a result, reliable data exchange is limited to the round trip delay of signals and the tolerance of the first piece of equipment to skew between clock and data.
In other known communication systems, serial data is transmitted between the first and second pieces of equipment using an asynchronous communication system. Asynchronous systems overcome the problem of round trip delays by requiring both the first and second piece of equipment to maintain separate time bases. The beginning of data transmission is signaled by pre-pending, or sending, a start bit from the transmitter to the receiver. The initiating edge of this start bit allows the receiver to lock phase with the start bit and recover subsequent bits which maintain phase with the start bit. As a result, reliable data exchange is maintained as long as the time bases of the first and second pieces of equipment have been set to the same frequency. In addition, the first and second pieces of equipment must maintain a small percentage drift between the two time bases between the time the start bit is transmitted and the time the final data bit associated with this start bit is received, otherwise the data will be improperly received.
As described above, these known methods of transmitting serial data each suffer from limitations in their ability to transmit data. These limitations often require a tradeoff analysis to be conducted to determine which of the two methods produce the maximum benefit.
It is desirable to provide apparatus and methods for transmitting serial data that uses a time base to determine the speed of data transmission and is not limited by cable delays.